Olivier Fabbri

Interactions between deformation and fluids in the frontal thrust region of the NanTroSEIZE transect offshore the Kii Peninsula, Japan: Results from IODP Expedition 316 Sites C0006 and C0007

Integrated Ocean Drilling Program (IODP) Expedition 316 Sites C0006 and C0007 examined the deformation front of the Nankai accretionary prism offshore the Kii Peninsula, Japan. In the drilling area, the frontal thrust shows unusual behavior as compared to other regions of the Nankai Trough. Drilling results, integrated with observations from seismic reflection profiles, suggest that the frontal thrust has been active since ∼0.78–0.436 Ma and accommodated ∼13 to 34% of the estimated plate convergence during that time. The remainder has likely been distributed among out-of-sequence thrusts further landward and/or accommodated through diffuse shortening. Unlike results of previous drilling on the Nankai margin, porosity data provide no indication of undercompaction beneath thrust faults. Furthermore, pore water geochemistry data lack clear indicators of fluid flow from depth. These differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape. In turn, fluid pressures may affect deformation. Well-drained, sand-rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe. Recent resumption of normal frontal imbrication is inferred from seismic reflection data. Associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region. This evidence consists of seafloor bathymetry, normal faults documented in cores, and low porosities in near surface sediments that suggest removal of overlying material. Overall, results provide insight into the complex interactions between incoming materials, deformation, and fluids in the frontal thrust region.

Seismic slip propagation to the updip end of plate boundary subduction interface faults: Vitrinite reflectance geothermometry on Integrated Ocean Drilling Program NanTro SEIZE cores

Seismic faulting along subduction-type plate boundaries plays a fundamental role in tsunami genesis. During the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) Stage 1, the updip ends of plate boundary subduction faults were drilled and cored in the Nankai Trough (offshore Japan), where repeated large earthquakes and tsunamis have occurred, including the A.D. 1944 Tonankai (Mw = 8.1) earthquake. Samples were obtained from the frontal thrust, which connects the deep plate boundary to the seafloor at the toe of the accretionary wedge, and from a megasplay fault that branches from the plate boundary decollement. The toe of the accretionary wedge has classically been considered aseismic, but vitrinite reflectance geothermometry reveals that the two examined fault zones underwent localized temperatures of more than 380 °C. This suggests that frictional heating occurred along these two fault zones, and implies that coseismic slip must have propagated at least one time to the updip end of the megasplay fault and to the toe of the accretionary wedge.

Arabia-Somalia plate kinematics, evolution of the Aden-Owen-Carlsberg triple junction, and opening of the Gulf of Aden

New geophysical data collected at the Aden‐Owen‐Carlsberg (AOC) triple junction between the Arabia, India, and Somalia plates are combined with all available magnetic data across the Gulf of Aden to determine the detailed Arabia‐Somalia plate kinematics over the past 20 Myr. We reconstruct the history of opening of the Gulf of Aden, including the penetration of the Sheba Ridge into the African continent and the evolution of the triple junction since its formation. Magnetic data evidence three stages of ridge propagation from east to west. Seafloor spreading initiated ∼20 Myr ago along a 200 kmlong ridge portion located immediately west of the Owen fracture zone. A second 500 kmlong ridge portion developed westward up to the Alula‐Fartak transform fault before Chron 5D (17.5 Ma). Before Chron 5C (16.0 Ma), a third 700 km‐long ridge portion was emplaced between the Alula‐Fartak transform fault and the western end of the Gulf of Aden (45°E). Between 20 and 16 Ma, the Sheba Ridge propagated over a distance of 1400 km at an extremely fast average rate of 35 cm yr−1. The ridge propagation resulted from the Arabia‐Somalia rigid plate rotation about a stationary pole. Since Chron 5C (16.0 Ma), the spreading rate of the Sheba Ridge decreased first rapidly until 10 Ma and then more slowly. The evolution of the AOC triple junction is marked by a change of configuration around 10 Ma, with the formation of a new Arabia‐India plate boundary. Part of the Arabian plate was then transferred to the Indian plate.

Progressive illitization in fault gouge caused by seismic slip propagation along a megasplay fault in the Nankai Trough

The question of whether coseismic ruptures along megasplay faults in accretionary prisms (i.e., large landward-dipping thrust faults branching from the plate boundary) reach the seafloor is critical for assessing the risk of tsunami disaster. However, samples from active megasplay faults have not previously been available. Here we present geochemical and mineralogical data of megasplay fault samples obtained from the shallow (

Regional seismicity and on-land deformation in the Ryukyu arc: Implications for the kinematics of opening of the Okinawa Trough

The stress field evolution and the kinematics of opening of the Okinawa Trough are investigated on the basis of earthquake focal mechanisms and structural data in the Ryukyu arc and the Okinawa Trough. Focal mechanisms show that the crust underlying the arc and the trough undergoes extension along two suborthogonal directions: a regional arc-perpendicular extension and a local arc-parallel extension. Both extensions are concurrent and related to the same regional stress field characterized by permutating horizontal σ2 and σ3 axes. Earthquake slip vectors reveal a southward motion of the Ryukyu arc with respect to the south China block. The current pole of opening of the Okinawa Trough is located around 16°N and 50°E. Fracture analysis in Okinawa island allows identification of three episodes of extension: a late Miocene N40°W to N20°E extension (episode I), a late Pliocene to early Pleistocene N20°E extension (episode II), and a latest Pleistocene to present-day N20°W extension (episode III). Episodes II and III are characterized by permutations between the two horizontal σ2 and σ3 axes. By synthesizing regional deformation data and by comparing the geometry of the deformation with analogue models of oblique rifting we reconstruct the kinematics of opening of the Okinawa Trough since the late Miocene. The direction of divergence of the Ryukyu arc has rotated clockwise from ∼N150°E in the late Miocene to nearly N-S today.

Coeval formation of cataclasite and pseudotachylyte in a Miocene forearc granodiorite, southern Kyushu, Japan

Abstract Cataclastic rocks and pseudotachylytes are exposed along the Uchinoura shear zone, a normal fault zone cutting the middle Miocene (14 Ma) Osumi granodiorite in southern Kyushu, Japan. Cataclastic rocks include non-foliated clast-supported to matrix-supported cataclasite and foliated clast-supported cataclastic granodiorite. In these rocks, fracturing and comminution played a major role, but dissolution and recrystallization of quartz, and plastic deformation of quartz and biotite were also active processes, especially in foliated granodiorite. Two types of pseudotachylyte are distinguished: a foliated-type characterized by a planar arrangement of clasts and microlites, and a spherulitic-type characterized by clasts surrounded by microlite overgrowths. Both types are of melt origin, as attested by the presence of microlites and rounded or embayed clasts, and by the scarcity of biotite clasts. Unlike spherulitic-type pseudotachylyte, which solidified without being deformed, the foliated-type pseudotachylyte underwent flow before complete solidification. This deformation is thought to reflect post-seismic strain accommodation immediately following the main slip episode. Kinematic indicators, which consist of Riedel-type secondary fractures branching on primary fractures, shear bands offsetting the foliation of foliated granodiorite, or asymmetrical porphyroclast systems within pseudotachylyte veins, show that all fault rocks were generated during N–S- to NW–SE-directed extensional deformation. Pseudotachylyte is closely associated both in time and space with cataclastic rocks, thus indicating that the behaviour of the Uchinoura fault zone alternated between comminution and frictional melting. Given the slow strain rates which characterize dissolution and recrystallization processes detected in cataclasites, the juxtaposition of pseudotachylytes and foliated cataclasites provides an example of aseismic and seismic displacements within the same shear zone.

The Miocene bending of Southwest Japan: new 39Ar/40Ar and microtectonic constraints from the Nagasaki schists (western Kyushu), an extension of the Sanbagawa high-pressure belt

Abstract In the westernmost part of Kyushu, the Nagasaki metamorphics (also observed in Amakusa) consist of oceanic sediments and fragments of ophiolite. These rocks underwent a high-pressure metamorphism followed by an increase in temperature up to the upper greenschist facies prior to the Late Cretaceous. Microtectonic and kinematic analyses indicate a synmetamorphic ductile shear from south to north. Dating of glaucophane, phengite and biotite by 39Ar/40Ar method suggests that the peak of metamorphism and the related deformation occurred approximately 90–95 Ma ago (i.e. Cenomanian), but a resetting by an Oligo-Miocene tectonic event is also evidenced. Due to discontinuous exposures, the metamorphic rocks of Kyushu cannot be continuously traced to the metamorphic belts of Honshu and Shikoku. Nonetheless the lithostratigraphy, metamorphic environment, radiometric data and deformation style strongly support a correlation of the Nagasaki metamorphics with the Green Schist nappe of Southwest Japan which underwent the Late Jurassic/early Cretaceous high-pressure Sanbagawa metamorphism. Such a correlation implies that (1) in the Amakusa area, the Nagasaki metamorphics form a window north of a meta-olistostrome that belongs to the Tanba-Ryoke zone; (2) the trend discrepancy of the stretching lineations between West Kyushu and Southwest Japan is in agreement with a clockwise rotation of the latter with respect to the former assumed to be fixed to Eurasia. Such a bending occurred after Paleogene times, probably during the opening of the Sea of Japan.

Neogene stress field in SW Japan and mechanism of deformation during the Sea of Japan opening

We present new structural data from SW Japan and discuss the mechanism of deformation during the opening of the Sea of Japan. We studied the Miocene basins at the southern margin of the Sea of Japan (northern coast of SW Japan) and of the Median Tectonic Line (MTL) by means of fault slip analysis to determine stress field directions during the opening of the Sea of Japan in early and middle Miocene time. On the southern margin, the stress field was extensional or transtensional with a NW-SE trending extension direction. The MTL was a normal fault in Shikoku during early middle Miocene and probably a normal fault with a left-lateral component in central Japan before having been strained by the collisions of the Tanzawa and Izu blocks with central Japan. We combine our results with published data for the Sea of Japan area and show that early and middle Miocene stress field directions are remarkably consistent on the eastern, southern, and western margins of the Sea of Japan with σHmax trending NE-SW and σHmin trending NW-SE. The stress regime is transpressional or transtensional on the eastern and western margin and almost purely extensional on the southern margin. This stress field distribution is in agreement with the model of the opening of the Sea of Japan in an extensional transfer zone between two N-S right-lateral strike-slip zones proposed by Jolivet et al. (1991). Analogue modelings and kinematic reconstructions showed that this model accounts for 20° to 30° of clockwise rotation in SW Japan, while paleomagnetic rotations reached 40° to 50° during the opening. We present structural data showing that SW Japan was strained during the rotation and did not behave as a rigid block. Our observations suggest that rotations of small blocks may have occurred. We propose that SW Japan has been sheared between the right-lateral strike-slip zones which bound the Sea of Japan and that the missing 10° to 30° of rotation are to be found in its internal deformation. In our model, the MTL is a normal fault with a strike-slip component rotating in a right-lateral shear zone.

Structural evolution of the Nojima fault (Awaji Island, Japan) revisited from the GSJ drill hole at Hirabayashi

Following the Hyogoken Nanbu earthquake (January 17, 1995, Mw=7.2), three drillholes were sunk through the Nojima Fault (Awaji Island, Japan). Textural and petrographic studies of the Geological Survey of Japan (GSJ) drill cores allow recognition of two deformation episodes. The first one is older than the deposition of the Middle to Late Eocene Kobe Group, corresponds to a left-lateral movement on the Nojima fault and is expressed by pseudotachylytes, kinking of biotite crystals in the low-strain rocks and an intense laumontite hydrothermal alteration. The second one displaces the basal unconformity of the Kobe group, corresponds to a right-lateral reverse displacement and is expressed at least by carbonate-filled hydraulic fractures and thin gouge zones. Different important deformation mechanisms are recorded by the fault rocks, but questions relating to the attribution of deformation and alteration features to one or other deformation episodes remain unresolved.

Post‐seismic permeability change in a shallow fractured aquifer following a ML 5.1 earthquake (Fourbanne karst aquifer, Jura outermost thrust unit, eastern France)

[1] Following a M L 5.1 earthquake in eastern France, a post-seismic 12-day long electrical conductivity increase was recorded in the water discharging from a karst aquifer located 3 km from the epicentre. We attribute this to a permeability enhancement which allowed long-residence time water from low-permeability fractures located in the saturated zone to be expelled. The permeability enhancement shows that shallow aquifers can be significantly deformed for several days by moderate magnitude earthquakes.